Gyms and fitness centers offer an array of exercise equipment; most provide a seated abdominal-crunch and torso-rotation machine for abdominal exercises. Although these machines allow you to add considerable resistance to the crunch exercise, they are no more effective at targeting your abdominal muscles than a regular crunch.
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Seated Abdominal Machine
A seated abdominal machine targets your rectus abdominis, which is your most visible ab muscle. It has an adjustable seat and a back pad for lower-back support. Your upper back rests against a shoulder pad that leans back past perpendicular to the floor to provide you with an extended range of motion. Move the weight pin in the weight stack to select your resistance. Sit in the machine and grab the handles. Crunch forward, pull your ribs toward your hips and allow your lower back to round. Keep your lower back pressed into the back pad throughout the movement.
The torso-rotation machine targets your obliques, or side abs. It allows you to twist your torso against resistance. Some torso-rotation machines hold your upper body stationary while your lower body rotates; others hold your lower body stationary while your upper body rotates. Sit in the machine and rest your chest against the chest pad. Pull your belly button in toward your spine to activate your abdominal muscles. Grab the handles and twist side to side in a smooth motion.
A study conducted at the University of Nebraska, Kearney, tested the effectiveness of a seated abdominal machine compared to a regular crunch. The subjects completed repetitions on the machine using resistance equal to 50 percent of their one-repetition maximum and 75 percent of their one-repetition maximum. The abdominal machine did not activate the rectus abdominis muscle significantly more than the standard crunch, researchers found. However, the machine did engage the rectus femoris, a muscle in the front of your thigh, considerably more. As the participants used more weight on the abdominal machine, the activity of the rectus femoris increased, but the activity of the abdominal muscles did not increase.
Although the abdominal machines at a gym might not work the abdominal muscles any more than a regular crunch does, this doesn't mean these machines don't have a place in your workout routine. If you have difficulty getting down to and up from the floor, you might find an abdominal machine much more convenient to use. The participants in the Nebraska study were athletes who knew how to properly use an abdominal machine. If you pull with your arms or use momentum, this decreases the effectiveness of the machine even further.